We will continue our project to induce paracentric and pericentric chromosomal inversions in different mouse strains and stocks. For each new inversion of considerable length, we will (1) determine its linkage group and genetic location on that chromosome, (2) mark it genetically if possible, or place it with a genetically marked homologous chromosome, (3) study its cytological, physiological, and anatomical effects, and (4) use it possibly in combination with other inversions or Robertsonian metacentric translocations, to produce test systems for precise estimates of mutational loads in irradiated populations. Specific proposed studies for this coming year are (1) to initiate the project to induce pericentric inversions, (2) to complete a study on the use of inversions to study somatic recombination which is itself increased by some mutagens, (3) to complete a study on the first lethal induced by a mammalian inversion system, (4) to determine the linkage and genetic locations of two newly induced inversions, (5) to complete chromosomal banding studies of existing inversions, and (6) to continue the process of placing two inversions in a single stock in order to examine a greater portion of the genome in mutagenesis studies.